It’ll use the Signal protocol, also used in WhatsApp, Facebook Messenger, and others.

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Since its inception, Skype has been notable for its secretive, proprietary algorithm. It's also long had a complicated relationship with encryption: encryption is used by the Skype protocol, but the service has never been clear exactly how that encryption was implemented or exactly which privacy and security features it offers.

Presently, Private Conversations are only available in the Insider builds of Skype. Naturally, the Universal Windows Platform version of the app—the preferred version on Windows 10—isn't yet supported. In contrast, the desktop version of the app, along with the iOS, Android, Linux, and macOS clients, all have compatible Insider builds. Private Conversations aren't the default and don't appear to yet support video calling. The latter limitation shouldn't be insurmountable (Signal's own app offers secure video calling). We hope to see the former change once updated clients are stable and widely deployed.

We've criticized Skype's failure to provide this kind of security in the past. Skype still has valuable features, such as its interoperability with traditional phone networks and additional tools for TV and radio broadcasters. But its tardiness at adopting this kind of technology left Skype behind its peers. The adoption of end-to-end security is very welcome, and the decision to do so using the Signal protocol, rather than yet another proprietary Skype protocol, marks a change from the product's history.

Although Skype remains widely used, mobile-oriented upstarts like WhatsApp and Facebook Messenger rapidly surpassed it. Becoming secure and trustworthy is a necessary development, but whether or not it's going to be sufficient to reinvigorate the application is far from clear.